A combine is a machine that is used in grain farming to harvest a variety of crops from a field. Combines are typically propelled and powered by an on-board engine, but some are also towed and powered by tractors. As a combine moves through a field during a harvesting operation, ripened crop is cut from the field by a header at the front of the combine. The crop is then transferred into threshing and separating assemblies within the combine, where grain is removed from the crop material. The resulting clean grain is stored in a grain tank located on the combine. The clean grain can then be transported from the grain tank to a truck, grain cart or other receiving bin by an unloading auger.
Header units may be configured in a variety of arrangements for harvesting different types of crops. Examples include those configured for harvesting corn (corn head) and those for harvesting cereal grains (grain head). Ordinarily, most of the functions of the header unit are driven mechanically, receiving power directly from the engine via a transmission. The transmission typically comprises: a fixed-speed or variable-speed belt-drive in combination with a further gear reduction, such as a planetary gear-train. In addition, hydrostatic motors may be employed to drive additional header functions, such as a gathering reel on the grain head.
Under normal harvesting conditions, the engine is preferably set at a fixed operating speed by the operator, with the mechanically driven functions of the header unit also operating at a constant fixed speed relative to engine speed. Conversely, the ground speed of the combine may be varied with operator control of the variable-speed hydrostatic transmission, irregardless of engine speed. Any hydrostatically driven functions of the header unit may also be varied independently by operator control, but are often configured to operate at variable speeds relative to ground speed, or at speeds proportional to the operation speed of mechanically-driven header functions.
With a constant-speed belt-drive in heavy harvesting condition, the increased load on the header unit causes the power output of the engine to increase in order to maintain a constant operating speed. If the load on the header becomes excessive and the power capacity of the engine at the fixed operating speed is exceeded, the engine will slow down and potentially stall unless the operator of the combine reduces the ground speed. To address this problem, a variable-speed belt drive may be employed in combination with load sensors that trigger the belt-drive ratio to be varied under heavy harvesting conditions, causing the header unit to slow down rather than draw excessive power from the engine. However, the reduced header speed will still prompt the operator of the combine to reduce ground speed, resulting in diminished harvesting productivity.